Fix code generation when the if branch has a value and there is no else branch

[ogoffart]

Fixes #4942

[ogoffart]

Currently the C++ test fails:

/tmp/.tmpS3RnNa.cpp: In lambda function:
/tmp/.tmpS3RnNa.cpp:71:46: error: statement has no effect [-Werror=unused-value]
   71 |     return [&]{ [&]() -> void { if (arg_0) { 45; } else { ; }}();return 12; }();
      |                                              ^~
cc1plus: all warnings being treated as errors

I wonder what to do:

1. ignore this test for c++ with the ignore: cpp comment
2. Somehow find a way to add -Wno-unused-value to that test only. How?
3. Add -Wno-unused-value to all C++ tests
4. workaround the warning somehow in the code.
5. delete all code that don't do anything in one of the optimization pass. With a warning that the code is useless in the .slint file

What do you think?
5 is the best. But I think short term i'll do the easy 1

[tronical]

I wonder do you can do the same as in the Rust code generator but instead of appending a semicolon prepending (void) like we used to do in Qt. (void)(someexpr)

But short of that yeah (1) seems the way to go.